Use Bootstrap tooltips instead of plain title attributes

By using data-original-title the tooltips live update without reapplying the
js code, such as .tooltip('fixTitle') each time the content changes. This
method also works well with angular expressions:

    data-original-title="{{'Download Rate' | translate}}"

This example provides a bootstrap tooltip saying 'Download Rate' that changes
automatically when the language is updated.

gui/default/syncthing/core/tooltipDirective.js

@@ -0,0 +1,11 @@
+angular.module('syncthing.core')
+    .directive('tooltip', function () {
+        return {
+            restrict: 'A',
+            link: function (scope, element, attributes) {
+                $(element).tooltip({
+                    html: 'true'
+                });
+            }
+        };
+});